1. Field of the Invention
The present invention pertains primarily to radiation curable adhesive compositions and more particularly to high energy ionizing radiation curable adhesive compositions containing no solvents, and their employment for the fabrication of certain composite structures.
2. Description of the Prior Art
The state of the art is believed to be indicated by the following cited references: U.S. Pat. No. 4,039,722--Dickie et al; U.S. Pat. No. 3,933,566--Seiberling; U.S. Pat. No. 3,924,021--Maruyama et al; U.S. Pat. No. 3,869,341--Gotoda et al; U.S. Pat. No. 3,697,397--Kehr et al; U.S. Pat. No. 3,485,732--D'Alelio; U.S. Pat. No. 3,338,810--Warner; U.S. Pat. No. 3,188,229--Graham; and U.S. Pat. No. 2,921,006--Schmitz et al.
Basically, there are four main sources of radiant energy: gamma rays, high energy electrons, neutrons, and ultraviolet. Each of these sources has its respective advantages and disadvantages. The use of radiant energy to cure polymeric coatings or adhesives is of fairly recent origin, and it is only in the last several years that the two most commercially attractive sources, ultraviolet and electron beam, have been developed for use in industry. Because high energy radiation curing is a new technology, there are many shortcomings such as limited production experience, high initial costs, and lack of coatings and adhesives adapted for use with ultraviolet or electron beam systems. Examples of typical ultraviolet applications include printing inks and can coatings. These utilize polymeric materials such as acrylated polyethers, acrylated polyester-based polyurethanes, methacrylated polyesters and acrylated epoxies. With respect to electron beam, there are only a few operations currently in use in this country with one of the largest being Ford Motor Company's Saline, Mich. plant which uses an electron beam system to cure coatings on plastic parts, chiefly dashboards.
Although both ultraviolet and electron beam systems require a blanket of inert gas to eliminate the formation of harmful ozone gas, and have many other similarities, an electron beam system differs from an ultraviolet system in two main ways. First, an electron beam source delivers a higher degree of penetration than an ultraviolet source. While this makes possible the curing of coatings of significantly greater thicknesses, an electron beam source requires a substantial capital investment since equipment costs are high. An ultraviolet source, on the other hand, allows for less penetration, necessitating curing of only thin layers of coating, but equipment costs are substantially lower. Second, compositions for UV curing require the presence of a photosensitizer or photoinitiator, such as peroxide, benzoin or benzophenone. These initiators add an estimated 15% to the cost of ultraviolet coatings as compared to electron beam coatings. Such additives are not required with an electron beam system thus resulting in less formulation work and reduced compound costs.
Electron beam curing systems also offer distinct advantages over conventional heat or chemical processing. Electron beam machines currently commercially available offer simple control of penetration, dose, and line speed. The usually rather compact machinery furthermore allows for both uniform irradiation of a specific product as well as an integrated processing capability in the presence of air, inert gases, etc. Single, multi-sample, or continuous conveyor feeds are available.
The basic chemistry of high energy ionizing radiation curing adhesives or coatings involves free radical formation of certain reactive species due to the radiant energy applied. Radiation intensity is, of course, a factor of prime importance. Reaction rate is a function of variables other than radiation intensity such as temperature (the Arrhenius equation is applicable for correlating the temperature dependence of the rate constant); concentration; location of double bonds; steric hindrance; resonance and inductive effects on the reactivity of the double bond; the size and molecular weight of the monomer; and the viscosity of the medium.
A wide variety of resins and monomers have been investigated for the high energy ionizing radiation curing of adhesives and coatings. The systems of greatest interest are based on a variety of polymerizable prepolymers and monomers. Polyesters and acrylics are the most common, but urethanes, alkyds, epoxies and silicones have likewise been investigated, albeit to a lesser extent. Recently, a considerable research and development effort has been directed towards development of radiation curable acrylic polymer and/or monomer systems. Acrylic materials have been prepared, for example, by converting vinyl copolymers of various monomers and glycidyl methacrylate to radiation curable products by reacting the pendant oxirane group with acrylic acid. Acrylic derivatives have also been made based on modifying urethanes and epoxy resins, including, for example, reaction of appropriate polymeric moieties with acrylyl chloride. The order of reactivity to high energy ionizing radiation as determined by prior art research proposes that multifunctional acrylates are more reactive than the corresponding methacrylates. However, none of the prior art adhesives or coating systems appear to be fully satisfactory for the preparation of such fiber flocked rubber and metal composite structures as those described herein.
To be most useful in radiation curing, adhesive compositions should have minimum dose sensitivity. In simple terms, dose sensitivity means that both the total dosage and the rate of applied dosage affect the cure rate. The desired non-dose sensitive adhesive systems will provide the same cure results if the same radiation dosage is applied as ten exposures of one unit each, one exposure of ten units, or twenty exposures of a half unit each because a non-dose sensitive system is affected by the total dosage only and is not affected by the size of individual doses administered.
It is a principal object of the present invention therefore to provide a new radiation curable adhesive composition for fabrication of certain composite structures.
It is also an object of the present invention to provide a new high energy ionizing radiation curable adhesive composition which is not dose rate sensitive and is curable at low total dosages for lowest cost.
It is a further object of the present invention to provide a new radiation curable adhesive composition capable of being cured by exposure to high energy ionizing radiation at ambient temperatures and in the presence of air for the fabrication of laminates consisting of fiber flocked rubber sheets and a metal base.
It is another object of the present invention to provide a new radiation curable adhesive composition comprised of specified blends of elastomers, chemically compatible ethylenically unsaturated monomers, tackifiers, adhesion promoters and optionally, pigments, fillers, thickeners and flow control agents.
A still further object of the present invention is to provide a new high energy ionizing radiation curable adhesive composition containing no solvent thereby eliminating the need to remove the solvent from the composition as part of the curing process.
Other objects, features and advantages of the present invention will become apparent from the subsequent description, and examples, and the appended claims taken in conjunction with the accompanying drawings.